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Abstract

Fixational eye movements remain a major cause of artifacts in optical coherence tomography (OCT)
images despite the increases in acquisition speeds. One approach to eliminate the eye motion is to
stabilize the ophthalmic imaging system in real-time. This paper describes and quantifies the
performance of a tracking OCT system, which combines a phase-stabilized optical frequency domain
imaging (OFDI) system and an eye tracking scanning laser ophthalmoscope (TSLO). We show that active
eye tracking minimizes artifacts caused by eye drift and micro saccades. The remaining tracking lock
failures caused by blinks and large saccades generate a trigger signal which signals the OCT system
to rescan corrupted B-scans. Residual motion artifacts in the OCT B-scans are reduced to 0.32
minutes of arc (~1.6 µm) in an in vivo human eye enabling acquisition of
high quality images from the optic nerve head and lamina cribrosa pore structure.

An overview of the information flow. The TSLO images the retina at 30 frames/s and the eye motion
is extracted in the TSLO-PC using the FPGA and GPU. The inverse eye motion signals are then scaled
to match the voltage range in the OCT system (Gain). Tracking signals are combined with OCT beam
steering signals in the electronic summing junction (+) to compensate for the eye
motion in real-time. The tracking validity signal is used to indicate B-scans that need to be
rescanned because tracking failures occurred due to large eye motions or blinks.

The magnitude of motion correction of the TOCT as a function of frequency. The plot was generated
from the eye motion measurement performance of the TSLO (Sheehy et al. [29]) combined with the additional error caused by the latency
between the eye motion measurement and the output of correction signals to the OCT galvo
scanners.

Residual motion that is present in the OCT data after applying the optimal gain setting for eye
motion compensation. The standard deviation is 0.37 minutes of arc, which corresponds to
approximately 2.7 µm (as comparison the spot size on the human retina was calculated to be
13.7 µm).

B-scans taken from the model eye over the course of 5 seconds. (A) B-scans were taken without
tracking and the features can be seen oscillating in the trace image. Below the image, all 250
frames were averaged which resulted in a blurry cross-sectional image of the tape layers in the
artificial retina. (B) The same location was imaged with tracking. It is clearly seen that the
previously seen motion is compensated. Again, below the image, all 250 frames were averaged together
and this produced a cross-sectional image of the tape layers with clear structure. (C) The first
frame from each data set was taken as reference frame and the consecutive frames were
cross-correlated to the selected reference. In C the horizontal B-scan motion is plotted as a
function of time. The red curve is derived from A (no tracking) and the blue curve from B
(tracking). The standard deviation for the untracked curve is 8.3 arcmin and for tracked 0.4 arcmin.
Scale bars indicate 0.5 deg.

En face (B-scans integrated over depth) images of the model retina consisting of
layers of tape under different conditions. (A) No motion is present and tracking signals are not
generated. (B) System is introduced to a horizontal 1 Hz sinusoidal motion with an amplitude of
±12.4 arcmin, tracking is off. (C) Motion is the same as in B but tracking signals are
generated and combined with OCT beam steering signals to compensate eye motion. In C it is seen that
the original structure of the moving retina is recovered. Scale bars indicate 0.5 deg.

B-scan trace image of 250 B-scans consisting of 2000 A-lines/B-scan taken without (A) and with
(B) tracking. (A) The left graph shows the eye traces extracted from TSLO videos where blue is the
horizontal motion and green is vertical. The B-scan trace image correlates with the blue curve
(horizontal motion). On the right, a cross-correlation graph of the OCT data is plotted, which
matches well with the TSLO horizontal eye trace. In the OCT cross-correlation plot, the first B-scan
of the data set was taken as a reference frame and consecutive frames were cross-correlated against
it. Only lateral motion was calculated, axial motion was ignored. (B) Same location as the area in
(A) but with tracking. A large saccade is present that is seen as a peak in the cross-correlation
curve and as a clear shift in the B-scan trace image. Scale bars indicate 0.5 deg.

En face images from 4 different volume data sets. (A) Large field of view
(10.6° or 3.11 × 3.11 mm) was imaged without tracking enabled. On the left of the
image A the corresponding eye traces are plotted. Below the image, three different areas are shown
as zoomed versions from the large image. (B) Same area imaged as in A but tracking was enabled.
Enlarged areas show that the motion artifacts are compensated. (C) The smaller field of view
(5.3° or 1.56 × 1.56 mm) clearly demonstrates eye motion artifacts. One large saccade
causes the scanning grid to acquire data from different position. (D) Same area imaged as in C but
with tracking enabled. In this data set there is a large saccade which is tracked well. However, the
final image still shows artifacts from brief tracking failures. To fully compensate large eye
motions, a validity signal needs to be used to rescan the areas that are affected by improper
tracking (see Fig. 10). Scale bars indicate 0.5 deg.

Tracked en face images with validity signal. When tracking software lost
tracking due to a large saccade or blink, OCT-PC was signaled to step back 10 B-scans and hold that
position until tracking was locked again on target. B-scans collected during tracking failure are
removed in post-processing and replaced with rescanned counterparts. (Top images)
En face of all acquired B-scans is shown. This includes B-scans acquired during
large saccades or blinks. The blink can be seen as a black line in the upper left image.
(Bottom images) Motion or blink corrupted B-scans are removed from the volume data
set. The black line caused by the blink is gone and several large saccades are also removed. Scale
bars indicate 1 deg.

C-scans extracted from different depths from the ONH movie (Low resolution:
Media 1
(3.9 MB), High resolution: Media 2 (17
MB)). Four different data sets were compounded together via image registration to enhance the SNR.
The laminar cribrosa mesh-like structure is clearly seen in all selected depths and the pore size
increases when moving further away from the center of the optic disc. On the left of the figure, a
B-scan is shown to illustrate at which depth each slice (C-scan) was taken (white dashed line
indicates the reference point). The porous structure of lamina cribrosa is still visible even at
depth of 429 µm measured from the bottom of the ONH cup.